Driving dynamics regulation serves to assist the driver in critical driving situations and to automatically stabilize the vehicle again. Known driving dynamics regulating systems, such as ESP (electronic stability program) or ROM (rollover mitigation), usually make use of the brakes of the vehicle or the engine management system as correcting elements in order to intervene in the driving operation. Other systems also use, for example, an active suspension/shock absorber system (normal force distribution system) or active steering.
Driving dynamics regulation, such as ESP, regulates in most cases the yaw velocity of the vehicle, i.e. the rotation of the vehicle about the vertical axis. When a vehicle oversteers or skids, the yaw velocity is higher than it ought to be based on the driver's inputs (steering wheel angle, accelerator pedal position, brake actuation). In order to stabilize the vehicle, the regulating algorithm calculates a compensating yawing moment which is implemented by activating selected wheel brakes. The regulating algorithm usually specifies for this a braking torque in the form of desired slip for individual wheels which is set using a brake-slip controller.
For setting a suitable compensating yawing moment in the case of oversteering, the wheel on the outside of the curve is particularly suitable. That wheel exhibits a favorable lever arm with respect to the center of gravity of the vehicle and is also capable of transmitting a high force based on a typical chassis configuration.
Driving dynamics regulating systems having a tipping stabilization function, such as ROM, also typically act, by braking interventions, on the front wheel on the outside of the curve. That wheel is in most cases under a high load and therefore greatly contributes to the development of high and potentially critical lateral acceleration.
In highly dynamic maneuvers, for example lane-changing or fishhook maneuvers, the vehicle typically enters a critical driving condition when the first counter-steering movement is performed. In that instance, on the one hand, high lateral acceleration may occur, causing a situation in which particularly vehicles with a high center of gravity may enter a critical range as regards tipping. On the other hand, a high degree of oversteer may also occur at that time. Braking torque intervention in the front wheel on the outside of the curve accordingly helps in highly dynamic maneuvers both to prevent tipping and to prevent oversteer.
Known driving dynamics regulating systems normally intervene in the driving operation when the control deviation of the yaw velocity exceeds a predefined regulation commencement threshold. When the regulation commencement threshold is exceeded, a correction request is sent to the hydraulic brake system, or rather to a hydraulic pump of the brake system, and various valves of a hydraulic unit are activated by the control unit.
Owing to various delaying factors, for example ramp-up of the hydraulic pump to nominal speed or filling of the brake with brake fluid, etc., it is possible for the brake pressure to be built up only with a finite gradient, however, with the result that the desired target braking torque is obtained only after a predefined period of time which is dependent on the brake system. That delay time may mean that, especially in highly dynamic maneuvers, vehicles may go into a skid and not be stabilized sufficiently quickly. Particularly in the case of vehicles with a high center of gravity, such as vans or SUVs (sports utility vehicles), the delayed response behavior of the brake system may lead to very high lateral acceleration which causes the vehicle to overturn.